1. Field Of The Invention
This invention relates to the blending of gases, and especially to the blending of calibration gases useful for the calibration of gas analysis instruments and the like. The present application contemplates certain alternative arrangements and further improvements as compared with the prior application above identified.
2. The Problem And The Prior Art
Calibration gases have been widely used in recent years for the calibration of instruments used in the analysis of mixtures of gases. The demand for calibration gases has become particularly active as a result of the need to accurately measure vehicle emissions as well as industrial stack emissions, chemical process streams, atmospheric pollutants, furnace atmospheres, life support atmospheres and the like. The need has been further stimulated by state and federal controls governing the emissions of carbon monoxide, hydrocarbons and other exhaust gas components and by various inspection and testing procedures which have evolved in order to make certain that motor vehicles meet the required emissions standards. Indeed, it is anticipated that in the very near future all automobiles manufactured will have to be tested as they leave the assembly line to make certain that the emission of the above pollutants is maintained within prescribed limits. It is also expected that automobiles now in use will be inspected periodically to determine whether or not they are being maintained in a proper state of tune so that they meet the same or similar high standards.
Present day techniques for gas analysis require the use of an instrument in which a so-called calibration gas having a composition which is precisely known is measured to produce an instrument response. One example of the manner in which this is accomplished involves the use of non-dispersive infrared analyzers. In these analyzers, the calibration gas is delivered to a cell through which a beam of infrared light is passed. The response of the instrument to the presence of the calibration gas, as indicated by the absorption of infrared energy, is observed and recorded. Thereafter the unknown gas is passed through the cell and similar observations taken. The concentration of the unknown gas may then be determined by observation of the instrument response and comparison with that obtained with the calibration gas.
With increasing need for these and similar instruments, the use of calibration gas has increased sharply in recent years. Typically, the calibration gas is supplied to the user in 43.6 liter cylinders which are pressurized to about 1500-2000 p.s.i. In preparing the gas mixtures, mixing is accomplished by the introduction of the components of the gas blend into the cylinder and pressurizing with the background gas such as nitrogen to the high pressure. The components of the blend behave differently at high pressures than they do under atmospheric conditions, not mixing well and exhibiting a tendency to stratify. This is apparently because the mean free paths of the various molecules are sufficiently shortened at the higher pressures so that the individual components do not diffuse readily. Mixing under high pressure is a time consuming and expensive process, involving prolonged heating and agitating of the cylinder in order to produce a commercially acceptable product. By way of example, mixing times by the prior art approach may be two hours or more in a typical case.
One drawback with the prior art batch approach described above arises because of the difficulties involved in precisely matching the mixture in one cylinder with that mixed at a previous time within the tolerances needed by industry. Since the measurement of exhaust emission often involves the use of computerized equipment associated with the analyzer, adjustments in the instrument and or in computer programs associated with and controlled by the instrument have to be made each time a new gas cylinder is used. This involves extra time and expense on the part of the user and is a potential source of error in the process of gas analysis.
Although laboratory techniques for blending gases at atmospheric pressures have been known in the art, so far as we are aware the use of these has been limited and there has been no recognition of the desirability of the production of gas blends in pressurized vessels, controlling the mixing at low pressures by a monitoring of the same blend of gases after they have been elevated in pressure.
Another prior art approach known to applicant involves the weighing in of components using a beam type scale on which the mixing vessel is weighed. Although accurate mixing of gas blends can be accomplished using this method, suitable beam type scales are extremely costly and great care must be taken to insure that external factors such as dirt, vibrations or the like do not result in errors in the weighing process.
A discussion of prior art methods for producing calibration gases may be found in NON-DISPERSIVE INFRA-RED GAS ANALYSIS, D. W. Hill and T. Powell, Plenum Press, New York, New York, 1968.